Deforestation has occurred for thousands of years, forests have been cut down for industries, homes, wood fire, logging, cattle and ship building (Brown et al 2014). Deforestation releases CO2 in the atmosphere and stops CO2 absorption, leading to an increase in climate change impacts. However, the restoration of trees could be used as a CDR geoengineering method to reduce CO2 emissions, as urged by COP21 agreements to reduce carbon sinks (Bailey 2015). This can be undertaken by afforestation and reforestation. Afforestation is the human growth of trees on land, which has not been forested in the past. Reforestation is the human growth of trees on land, which was previously forested (Figure 1) (Caldeira et al 2013). The natural absorption of CO2 is stored in trees and reduces atmospheric CO2, hence this geoengineering process could reduce climate change impacts.
|Figure 1: Afforestation and Reforestation Processes|
Afforestation and Reforestation Impacts
The increase in tree plantation may be very beneficial as large amounts of CO2 will be absorbed from the atmosphere, provided trees are planted in the correct areas. Albeit reforestation and afforestation absorb large amounts of CO2 they may negatively impact aquifers, due to high demands of water for tree growth. Large amounts of trees require large amounts of water consumption for irrigation. Moreover, evapotranspiration levels will increase leading to an increase in aquifer discharge (Heck et al 2015). Hence although atmospheric CO2 may be absorbed and reduce climate change impacts, aquifers may deplete due to not enough recharge occurring in arid areas. Moreover, ecosystems may change due to changes in water availability. In China, there were abrupt changes in ecosystems due to a high level of tree planting and a low rainfall occurrence of 400-500mm per year (Brown et al 2014). This lead to a degradation of ecosystems due to not enough water being supplied, questioning the degree of benefits of afforestation and reforestation. Controversially, precipitation levels should increase by 1-2%, as bare soil will be replaced by trees, for instance in Nigeria. Abiodum et al (2013) suggest that extreme rainfall events may increase in Nigeria due to a slower monsoon occurrence created by afforestation. It is suggested that an increase in afforestation may have regional positive impacts of increased rainfall events, yet global warming may increase in regions around the afforested area. Additionally, there may be an increase in droughts in semi-arid regions, as monsoons decrease and delays in air moisture occur in the region, hence delaying rainfall events. It is evident that precipitation events will change due to afforestation and reforestation. However, it is unclear to what degree this may be beneficial or disadvantageous for the hydrological cycle.
Although afforestation and reforestation can reduce atmospheric CO2 a large supply of trees will also demand a high supply of nutrients. As afforestation and reforestation will occur at large scales, soil depletion will take place (Heck et al 2015). It is estimated that 50-150kg N/ha/yr will be required for herbaceous plants (Heck et al 2015). Thus a Nitrogen(N) fixation will be required to replenish the soils. It is likely that fertiliser use will increase due to an increase in nutrient demand for soil replenishment, hence increasing the costs of afforestation and reforestation (Heck et al 2015). Therefore, questioning the degree of this geoengineering process being advantageous.
Moreover, another negative impact that may occur with afforestation and reforestation may be a decrease in albedo (i.e. solar reflectance) and an increase in roughness to a higher coverage of land with trees (Heck et al 2015). Hence, if afforestation and reforestation occur on a large-scale, this may increase global temperatures at a local and global scale. Thus, it may not be as influential as desired as a geoengineering process due to not reducing climate change impacts .
Furthermore, there may be shifts in biodiversity, a main concern regarding environmental sustainability issues. To minimise this impact it is essential when planting trees in an area to maintain the local species (Heck et al 2015). Therefore plantation management is essential to ensure an equal growth of all species in a habitat (Heck et al 2015). It is very difficult to ensure that no species outcompetes another species causing a shift in food webs or the natural biodiversity of an area.
Lastly, there may be an issue when regarding space. If trees are planted at large scales for afforestation and reforestation, this creates an opportunity cost which enables the growth of agriculture or urban areas (Heck et al 2015). Hence, this may lead to a degradation of economic growth and food security (Caldeira et al 2013). Thus questioning to what degree people and governments would be willing to ‘sacrifice’ land for this geoengineering process to reduce CO2 levels.
Afforestation and reforestation can be considered an easy process to implement, with low costs and relatively low risks due to less technology being required and being such a familiar process. However, this process may occur at a fast rate and may degrade ecosystems and the water cycle. Hence it is questionable to what degree this process would be beneficial as a CDR method. I believe the growth of trees is always beneficial. However, I think the human induced growth of trees (especially in large-scales) would be very difficult to implement successfully, due to water and biodiversity management issues. I look forward to you opinion on this CDR method.